Its the major selling point of almost every event or even arc/storyline. So, when did this actually became true?
The one I remember the most is proly Emerald Twilight. Still not clear after what retcons what really happened but with that it introduced Kyle Rayner. No more yellow weakness/impurity. New uses of the ring. Resurrection of the Corps, etc. most of that still in continuity. Death of Superman and Knightfall didnt even came close to that. And the Crisis/rebirths keep rebooting everything to the same status quo so almost nothing ever changes.
Then there are things like Death in the Family/Jasson Todd back from the death. Tedd Kord murder/new Blue Beetle but I dont know what is canon or not. Do I even want to mention Barry Allen/Wally West?
And I guess it could also apply to things like (for better or worse) the sexification of Harley Quinn but im not aware how/when this happened, since I dont read batman much. The DC wasnt the same again after that thats for sure.